It is known that chromatin, which is a complex containing DNA and proteins, is present in most eukaryotic cells, and histones (proteins), which are the structural proteins of chromatin, play an important role in gene expression. DNA is entangled with histones to form a chromatin structure and it is said that acetylation of the histone tail of these histones causes a change in chromatin structure (as a result, resulting in gene expression regulation).
More specifically, chromatin is formed by higher order structure formation from a fundamental unit, the so-called nucleosome structure, formed by gene DNA winding around a core histone octamer resulting from association of two molecules each of four kinds of histones. The core histone has a tail-like form in the vicinity of the N terminus thereof which is rich in basic amino acids and has a structure further covering the DNA on the nucleosome. A lysine residue in the vicinity of this tail region is under turnover involving reversible acetylation and is said to be closely involved in structural regulation of the nucleosome itself or in transcriptional control via controlled binding thereof to other proteins (transcription factors, silencer proteins, RNA polymerase, etc.) interacting with the gene DNA.
As a proof of histone acetylation-dependent gene expression control, it is reported that a high degree of histone acetylation promotes the induction of expression from the gene occurring in that region and, on the contrary, deacetylation thereof results in the formation of a transcription-inert region called heterochromatin. Thus, in spite of the fact that histones, which are structural proteins of chromatin, and acetylation thereof extend to all parts of the chromosomal gene, it is suggested that the function thereof exerts a great influence on the expression of certain specific genes and thus is involved in strict control of the so-called nuclear signal transduction. The enzyme effecting the acetylation of histones is histone acetyltransferase and the enzyme effecting deacetylation is histone deacetylase (HDAC) and these enzymes both control the dynamic turnover with respect to the level of histone acetylation.
When the action of histone deacetylase is enhanced, the adequate differentiation of cells or morphological normalization thereof is inhibited; when the enzyme activity of this histone deacetylase is inhibited, the deacetylation of histones is inhibited and, as a result, a high level of histone acetylation is induced and the expression of genes necessary for differentiation and morphological normalization is induced. This phenomenon has been confirmed by studies using trichostatin A and trapoxin analogs, which are histone deacetylase inhibitors (HDAC inhibitors); more specifically, trichostatin A is known to induce the differentiation of leukemia cells, nerve cells, breast cancer cells and so forth. In addition, when these inhibitors are caused to act on cells at still higher concentration levels, the protein p21 inhibiting the cyclin-dependent protein kinase (CDK) is expressed and the cell cycle is inhibited thereby, resulting in proliferation inhibition. Therefore, certain HDAC inhibitor species are considered to serve as drugs causing cell differentiation or morphological normalization and the development thereof as anticancer agents has been attempted (cf. Non-Patent Documents 1 and 2). It is known that such apoptosis-inhibiting proteins as survivin, Bcl-xL and Bcl-2 are expressed at high levels in many cancer cells, and the cell death due to starvation stress caused by excessive proliferation or cell-damaging stress caused by radiation or an anticancer agent is thereby avoided. Certain HDAC inhibitors are known to reduce the expression of such apoptosis-inhibiting proteins and promote the death of cancer cells.
On the other hand, HDAC inhibitors are expected not only as anticancer agents but also as cancer-preventing drugs. It is reported that trichostatin A, suberoylanilide hydroxamic acid (SAHA) and the like markedly suppressed the occurrence of breast cancer in chemical agent-induced cancer animal models. From a study using valproic acid, it is also known that HDAC inhibitors suppress cancer metastasis (cf. e.g. Non-Patent Document 3).
Further, aside from cancers, it has been suggested in recent years that abnormal epigenetics resulting from chemical modification of chromatin might be involved in the causes of diabetes, rheumatism and like symptomatic diseases, autoimmune diseases, infectious diseases, neurodegenerative diseases and other diseases, the incidences of which increase with aging. Histone deacetylases are enzymes playing a core role in epigenetic control and are involved in the onset of diseases via various types of gene expression. Therefore, skilled molecular designing of HDAC inhibitors may possibly lead to development of medicaments not only for cancer but also for various epigenetic abnormality-due diseases, such as diabetes and other diseases mentioned above; thus, various applications have been attempted.
There are ten or more histone deacetylase subtypes and, in recent years, it has become known that there is a close relationship between certain specific histone deacetylase subtypes and cancer. For example, it has been made clear that the tumor suppressor gene p53 playing a very important role in suppressing carcinogenesis requires acetylation of p53 itself for its performing that function (cf. Non-Patent Document 4), that HDAC1 or HDAC2 is involved in inhibiting that function (cf. Non-Patent Document 5) and that the proteins PML-RAR and PLZF-RAR involved in the onset of acute promyelocytic leukemia (APL) and Bcl-6 and like oncogenes involved in the onset of lymphoma recruit HDAC4, among others, via a nuclear corepressor and thus suppress the expression of a group of genes necessary for normal differentiation, leading to carcinogenesis (cf. e.g. Non-Patent Document 6). On the other hand, it is known that among histone deacetylase subtypes which are expressed in a tissue-specific manner, there are some playing an important role in normal tissue development or differentiation (cf. Non-Patent Document 7).
HDAC6 is an enzyme shuttling between the nucleus and cytoplasm by nuclear export and generally localized in cytoplasm. HDAC6 is expressed at high levels in the testis and so forth and is possibly involved in the differentiation of normal tissues. HDAC6 is also known to be involved in deacetylation of microtubules and control the stability of microtubules. Further, HDAC6 is a deacetylase binding to microtubules and is involved in cell motility. Therefore, HDAC6 inhibitors may serve as metastasis inhibitors.
Histone deacetylases are hydrolases having a zinc atom at the active center thereof and the known agents inhibiting them mostly contain a hydroxamic acid group or a thiol group as a ligand for zinc. Therefore, the structural variety of the inhibitors is restricted and specific inhibitors capable of distinguishing histone deacetylase subtypes from one another have been developed only to an unsatisfactory extent. As regards inhibitors resulting from introduction, into cyclic tetrapeptides, of an atomic group forming a coordination bond with the zinc ion at the active center of histone deacetylases, several findings and proposals have been reported. As regards novel compounds having histone deacetylase-inhibiting activity, there are a number of patent documents concerning cyclic tetrapeptide derivatives, for instance (cf. Patent Documents 1-7 and Non-Patent Document 8). Among them, some compounds synthesized have potent enzyme-inhibiting activity; they are, however, not satisfactory from the viewpoint of toxicity or continued administration characteristics (stability against metabolism and absorbability), and none has been put into practical use as a medicament.    Patent Document 1: Japanese Patent No. 3494624    Patent Document 2: Japanese Kokai (laid-open) Publication 200-256397    Patent Document 3: Japanese Kohyo (laid open under PCT) Publication 2002-527449    Patent Document 4: Japanese Kohyo Publication 2003-505417    Patent Document 5: Japanese Kohyo Publication 2005-517683    Patent Document 6: WO 2003-70754    Patent Document 7: WO 2004-113366    Non-Patent Document 1: Nakajima, H., Kim, Y. B., Terano, H., Yoshida, M., and Horinouchi, S. (1998) FR901228, a potent antitumor antibiotic, is a novel histone deacetylase inhibitor. Exp. Cell Res. 241, 126-133    Non-Patent Document 2: Saito, A., Yamashita, T., Mariko, Y., Nosaka, Y., Tsuchiya, K., Ando, T., Suzuki, T., Tsuruo, T., and Nakanishi, O. (1999) A synthetic inhibitor of histone deacetylase, MS-27-275, with marked in vivo antitumor activity against human tumors. Proc. Natl. Acad. Sci. USA 96, 4592-4597    Non-Patent Document 3: Gottlicher, M., Minucci, S., Zhu, P., Kramer, O. H., Schimpf, A., Giavara, S., Sleeman, J. P., Lo Coco, F., Nervi, C., Pelicci, P. G., and Heinzel, T. (2001) Valproic acid defines a novel class of HDAC inhibitors inducing differentiation of transformed cells. EMBO J. 20: 6969-6978    Non-Patent Document 4: Ito, A., Lai, C. H., Zhao, X., Saito, S., Hamilton, M. H., Appella, E., and Yao, T. P. (2001) p300/CBP-mediated p53 acetylation is commonly induced by p53-activating agents and inhibited by MDM2. EMBO J. 20, 1331-1340    Non-Patent Document 5: Juan, L. J., Shia, W. J., Chen, M. H., Yang, W. M., Seto, E., Lin, Y. S., and Wu, C. W. (2000) Histone Deacetylases Specifically Down-regulate p53-dependent Gene Activation. J. Biol. Chem. 275, 20436-20443    Non-Patent Document 6: Dhordain P., Albagli, O., Lin, R. J., Ansieau, S., Quief, S., Leutz, A., Kerckaert, J. P., Evans, R. M., and Leprince, D. (1997) Corepressor SMRT binds the BTB/POZ repressing domain of the LAZ3/BCL6 oncoprotein. Proc. Natl. Acad. Sci. USA 94, 10762-10767    Non-Patent Document 7: McKinsey, T. A., Zhang, C. L., Lu, J., and Olson, E. N. (2000) Signal-dependent nuclear export of a histone deacetylase regulates muscle differentiation. Nature 408, 106-111    Non-Patent Document 8: Furumai, R., Matsuyama, A., Kobashi, N., Lee, K.-H., Nishiyama, M., Nakajima, H., Tanaka, A., Komatsu, Y., Nishino, N., Yoshida, M., and Horinouchi, S. (2002) FK228 (depsipeptide) as a natural prodrug that inhibits class I histone deacetylases. Cancer Res. 62, 4916-4921